514 



THE AUEL'H AX XATURAL1ST 



[Vol. XLTV 



theory of antithetic alternation of generations. The 

 higher plants were naturally the first which were sub- 

 jected to critical cytological investigation and the struc- 

 ture and behavior of the nuclei through various stages 

 of ontogeny have been studied in an increasing number 

 of forms representative of the most important groups. 

 Later came the pioneer work of this character upon the 

 thallophytes which has given such rich returns in our 

 appreciation of the significance of the critical stages in 

 the complex life histories of certain types of the Asco- 



It is the writer's part in this symposium to outline the 



the sexual reproduction of the algae, and Buch enquiry, 

 for a number of reasons, necessarily leads at once to the 

 consideration of life histories with reference to cytolog- 

 ical detail. From studies upon the bryophytes, pterido- 

 phytes and spermatophytes we have learned that in these 

 groups everv sexual nuclear fusion marks the beginning 

 of a. distinct phase in the life history, the sporophyte, the 



we know that the sporophytic phase comes to an end with 

 the numerical reduction of the chromosomes to the orig- 

 inal, or haploid, number peculiar to the gametophyte. 

 This significant period of chromosome reduction is asso- 

 ciated in the bryophytes and pteridophytes with the for- 

 mation of spores, in the spermatophytes with the forma- 

 tion of pollen grains and embryo sacs. Every sexual 

 fusion, then, carries with it a prospective period of 

 chromosome reduction. 



Since such is the history for the higher plants we have 

 every reason to expect that each sexual nuclear fusion 

 among the thallophytes with the resulting doubling of the 

 chromatin content in the zvgote would lie followed sooner 

 or later by reduction phenomena. It becomes a matter 

 of importance to determine when the latter event occurs, 

 since between the time when the gametes fuse and that of 



